This invention relates to a process for producing a granular, basic phosphate fertilizer. More particularly, it relates to a process for producing a granular, basic phosphate fertilizer which is difficult to disintegrate and difficult to dust against a physical stress such as by transport or mechanical fertilization operation and which exhibits an effect on improving acidic soil.
In countries where the climate is mild and rainy, bases are readily leached off from soil, permitting acidic soil to be distributed all over the country. Since the acidification of soil not only brings about a lowering of utilization rate of fertilizer components by crop, but also causes physical disorders of various crops, there have been developed a diversity of acidic soil improvers.
As for the fertilization effect in acidic soil, phosphate fertilizer may be typical fertilizer components which are sharply lowered in their effect. Various types of phosphate fertilizer have been thus proposed so as to improve the utilization rate. In this sense, a basic phosphate fertilizer showing the effect on improving acidic soil besides other desirable properties may be said to be one of desirous types of fertilizer.
Mentioned as one of basic phosphate fertilizers which are now commercially available is a fused phosphate fertilizer. As is well known in the art, the fused phosphate fertilizer is an amorphous powdery basic phosphate fertilizer obtained by thermally fusing a phosphate ore and a SiO.sub.2 and MgO containing material such as serpentine and rapidly cooling the fused components with cold water. This fused phosphate fertilizer is in most cases applied as a straight fertilizer because of a high estimation of its improving effect on the acidic soil. However, as the cost of energy rises sharply in recent years, the production of the fused phosphate fertilizer using a fusing process has become poorer in economy year by year, which makes its cost relatively higher than that of other fertilizers in relation to the unit of its available P.sub.2 O.sub.5.
In its physical aspects, the fused phosphate fertilizer has rather heavy density (an apparent density of 1.5-1.7) and is in the form of vitreous powder, so that there have been pointed out from long ago several disadvantages in handling, based on the inherent physical nature of products, such as a difficulty of uniform spraying in large amount and a problem of dust scattering of the vitreous powder on the spraying. In order to overcome these disadvantages, there have been proposed granulation processes such as in Japanese Patent Publication Nos. 39-3022, 39-22916 and 42-25179 but they have not been yet put into practice, leaving the problems unsettled as before. Thus, there does not exist any granular, basic phosphate fertilizer which is excellent in economy, fertilization effect, and handling characteristics along with an excellent effect on improving acidic soil. Accordingly, it is highly desired to realize the production of a fertilizer of the just-mentioned type as early as possible.
We have pursued the extensive studies to economically produce a granular, basic phosphate fertilizer which is easy to prepare, excellent in handling characteristics and high in effect of phosphate fertilizer together with a effect on improving acidic soil. As a result, it has been found that a granular, basic phosphate fertilizer can be produced advantageously from an industrial standpoint by a process which comprises mixing a secondarily produced basic material, such as slag, containing acid-soluble CaO and/or acid-soluble MgO and acid-soluble SiO.sub.2 which phosphoric acid and/or acidic phosphate in a specific ratio, and granulating and reacting the mixture at the same time. The term "acid-soluble" used in this specification and claims means "soluble in the aqueous N/2-HCl solution". The fertilizer obtained by the process is hardly susceptible to disintegration and dusting when undergoing physical stresses caused by, for example, transport or mechanical applications of fertilizer, excellent in handling characteristics, and high in effect of phosphate fertilizer, and has an effect on improving acidic soil. The present invention is accomplished on the basis of the above finding.